1. Field of the Invention
The invention relates to a method of and device for determining a spin resonance distribution from unequal numbers of spin magnetic resonance signals acquired for different values of the time integral of a phase encoding gradient in which are generated in a region of an object containing atomic spins, such as nuclear spins, the spin magnetic resonance signals acquired for the same value of the time integral are averaged after signal acquisition and before determination of the spin resonance distribution. The invention is particularly suitable to nuclear spin magnetic resonance but can also be used for electron spin resonance.
2. Description of the Prior Art
Such a method is known from British Patent Application No. 2,193,320 corresponding to U.S. Pat. No. 4,740,749. This application describes rf pulse and gradient sequences for NMR (nuclear magnetic resonance) in which over a range of values of the time integral over a first magnetic field gradient used for phase encoding not all values are chosen an equal number of times for the acquisition of spin magnetic resonance signals in order to obtain an image therefrom. Hereinafter, one pulse and gradient sequence will be referred to as a profile. Prior to the determination of the image, for example by way of Fourier transformation, the spin resonance signals of equivalent profiles, i.e. profiles involving the same values for the first magnetic field gradient, are averaged. Because of the non-uniform profile distribution, this is referred to as "selective averaging". In said British Patent Application profile distributions are chosen which offer, selectively over a given range of spatial frequencies, an improvement of the signal-to-noise ratio in the image. Because not all profiles are measured equally frequently for the improvement of the signal-to-noise ratio in the image, unnecessary prolongation of the overall measuring time required for obtaining resonance signals for an image is avoided. A drawback of the method disclosed in said British Patent Application consists in that in advance a profile selection is made which is not based on an unambiguous criterion. For example, a distribution is proposed in which profiles with comparatively small values of the time integral (corresponding to comparatively low spatial frequencies) occur more frequently. Generally speaking, profile distributions are proposed in which profiles for a predetermined range of spatial frequencies are measured more often.